Preceramic compositions and ceramic products

ABSTRACT

Preceramic polymer dispersions which have particular utility in providing protective ceramic coatings on carbon/carbon composites, graphite, carbon fibers, and other normally oxidizable materials are prepared by dispersing about 0-3 parts by weight of aluminum-silicon eutectic, about 0-4 parts by weight of silicon carbide, about 1.5-5 parts by weight of silicon boride, and about 0.4-5 parts by weight of silicon metal in a solution of an organoborosilazane polymer in an organic solvent, the organoborosilazane polymer being the product obtained by reacting about 0.25-20 parts by weight of a trialkoxy-, triaryloxy-, or tri(arylalkoxy)boroxine with one part by weight of a polysilazane.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser. No. 261,104, filed Oct. 24, 1988.

FIELD OF INVENTION

This invention relates to ceramic materials derived from polysilazanes and more particularly to such materials which are useful in protecting substrates that are normally susceptible to oxidative deterioration.

BACKGROUND

It is known that many materials, such as carbon/carbon composites, carbon fibers, graphite, and certain metals, have properties which make them attractive for use in aerospace and other applications in which their susceptibility to oxidative deterioration at elevated temperatures is a serious disadvantage. It would be desirable to find a means of protecting those materials from oxidation at high temperatures, and it has been proposed to provide such protection with ceramic coatings. However, known ceramic coatings have proved to be inadequate.

As disclosed in U.S. Pat. Nos. 4,397,828 (Seyferth et al.-I), 4,482,669 (Seyferth et al.-II), 4,645,807 (Seyferth et al.-III), 4,650,837 (Seyferth et al.-IV), and U.S. Pat. No. 4,659,850 (Arai et al.), it is known that ceramics can be obtained from polysilazanes. U.S. Pat. No. 4,482,689 (Haluska) discloses boron-containing metallosilazane polymers which are also useful in forming ceramic materials.

THE INVENTION

It has now been found that polymer compositions which can be converted to ceramic coatings capable of protecting oxidizable substrates from oxidative deterioration at elevated temperatures can be obtained by intimately mixing (A) a homogenized mixture of about 0-3 parts by weight of aluminum-silicon eutectic, about 0-4 parts by weight of silicon carbide, about 1.5-5 parts by weight of silicon boride, and about 0.4-5 parts by weight of silicon metal with (B) a solution of an organoborosilazane polymer in an organic solvent, the polymer being the product obtained by reacting about 0.25-20 parts by weight of a trialkoxy-, triaryloxy-, or tri(arylalkoxy)boroxine with one part by weight of a polysilazane, to form a dispersion.

The organoborosilazane polymers employed in the practice of the invention are disclosed in copending applications Ser. No. 242,493 (Niebylski-I), filed Sep. 9, 1988, and Ser. No. 272,481 (Niebylski-II), filed Nov. 17, 1988.

As taught in the copending applications, the polysilazane which is reacted with the boroxine may be any polysilazane that is soluble in common organic solvents, such as aliphatic or aromatic hydrocarbons, dialkyl or alicyclic ethers, etc.; and it may be, e.g., a polysilazane of Seyferth et al.-I, Seyferth et al.-II, Seyferth et al.-III, Seyferth et al.-IV, or Arai et al., the teachings of all of which are incorporated herein in toto by reference. However, it is preferably a polysilazane of the type taught by Seyferth et al.-II, i.e., a polysilazane prepared by reacting an organodihalosilane with ammonia, treating the ammonolysis product with a basic catalyst which is capable of deprotonating an NH group that is adjacent to an SiH group, and quenching the resultant product with an electrophilic quenching reagent, a mixture of such polysilazanes, or, alternatively, an oligomeric ammonolysis product formed as an intermediate in the process of Seyferth et al.-II and isolated as in Seyferth et al.-I. For example, it may be one or more polysilazanes prepared by reacting methyldichlorosilane with ammonia, treating the ammonolysis product with potassium hydride, and quenching the resultant product with methyl iodide or dimethylchlorosilane; or it may be one or more polysilazanes prepared by reacting methyldichlorosilane with ammonia and isolating the ammonolysis product.

The boroxine reactant is generally a compound corresponding to the formula: ##STR1## wherein R is an alkoxy, aryloxy, or arylalkoxy group, preferably an alkoxy, phenoxy, alkylphenoxy, phenalkoxy, or alkylphenalkoxy group in which any alkyl or alkoxy group contains 1-6 carbons, such as the trimethoxy-, triethoxy-, tripropoxy-, tributoxy-, tripentoxy-, trihexoxy-, triphenoxy-, tritolyloxy-, tri(2-ethylphenoxy)-, tribenzyloxy-, triphenethoxy-, tri(3-phenylpropoxy)-, tri(4- phenylbutoxy)-, tri(5-phenylpentoxy)-, and tri(6-phenylhexoxy)boroxines, the corresponding triphenalkoxyboroxines having non-linear alkyl chains, tritolylethoxyboroxine, etc. It is preferably trimethoxyboroxine or triphenoxyboroxine.

Regardless of the particular boroxine used, the amount employed is about 0.25-20 parts per part by weight of the polysilazane. However, when the boroxine is a trialkoxyboroxine, it is generally preferred to use about 1-6, most preferably about 3-4 parts per part by weight of polysilazane; and, when the boroxine is a triaryloxyboroxine, it is generally preferred to employ about 1-10, most preferably about 6-8 parts per part by weight of polysilazane.

To prepare the organoborosilazane polymers, the neat boroxine reactant (if sufficiently low melting) or a solution thereof in an organic solvent is added to a neat polysilazane (if a liquid) or to a solution of a polysilazane in an organic solvent to initiate an exothermic reaction which is preferably controlled to a temperature below 50° C. for a period of time sufficient to allow the formation of an organoborosilazane polymer. In a preferred embodiment of the invention, the polysilazane is used as a clear solution having a solids content of about 10-40%, preferably about 30% by weight; and the total amount of solvent employed is such as to provide an organoborosilazane polymer solids content of about 5-75%, preferably about 30-60% by weight.

The solvent employed for the boroxine and/or polysilazane may be any suitable organic solvent, such as hexane, heptane, and other aliphatic hydrocarbons; benzene, toluene, xylene, and other aromatic hydrocarbons; cyclohexanone, 1-methyl-2-pyrrolidone, and other ketones; etc.; and mixtures thereof. When it is desired to use a mixture of solvents for the reaction, the desired mixture may be introduced as the solvent for the polysilazane or for both the polysilazane and the boroxine, or different solvents may be used for the polysilazane and the boroxine.

The solids which are intimately mixed with the organoborosilazane polymers to form the dispersions of the invention are constituted by about 0-3 parts by weight of aluminum-silicon eutectic, about 0-4 parts by weight of silicon carbide, about 1.5-5 parts by weight of silicon boride, and about 0.4-5 parts by weight of silicon metal per part by weight of polysilazane employed in making the organoborosilazane polymer. The silicon carbide is preferably α-silicon carbide, the silicon boride may be silicon tetraboride and/or silicon hexaboride, and the silicon metal is preferably amorphous.

In the preparation of the dispersions, it is preferred to premix the silicon boride, silicon metal, and any aluminum-silicon eutectic and/or silicon carbide, homogenize and dry them, and then intimately intimately mix them with the organoborosilazane polymer solutions. Generally, the organoborosilazane polymer solutions are added to the homogenized solids, whether predispersed or not, and the resultant dispersions are agitated until they are uniform.

When the homogenized solids are predispersed in a organic medium, the amount of organic medium used is generally such that the ultimate dispersion has a total solids content of about 5-75% by weight, preferably about 30-60% by weight, if the dispersions are to be used as coating and/or infiltration materials.

The organoborosilazane polymer dispersions of the invention are preceramic materials which are useful for making ceramics such as coatings, structural composites, etc.; and, like other preceramic materials, they may be used in combination with other ingredients, such as ceramic powders or whiskers, etc., when appropriate.

An application in which they find particular utility is as coating compositions for normally oxidizable materials, especially those which need protection from oxidative deterioration at elevated temperatures. (Such materials include, e.g., fibers, tows, hanks, mats, and composites of carbon; carbon or graphite slabs, rods, and structures; and oxidizable metals, such as magnesium, aluminum, silicon, niobium, molybdenum, lanthanum, hafnium, tantalum, tungsten, titanium, and the metals of the lanthanide and actinide series.) When used in such an application in which the substrate is porous, the compositions also serve as infiltrants.

In addition to providing protection from oxidative deterioration, the coating compositions can also serve to improve the physical properties and thermal stability of substrates, such as those mentioned above, silica foams, ceramic cloths (e.g., cloths formed from alumina, silica, and/or lithia), etc.

The coating compositions are also useful for patching ceramic coatings formed from the same or different formulations.

When the dispersions are to be used to provide protective ceramic coatings on substrates, the surfaces to be coated are usually cleaned prior to the application of the coating composition in order to improve the bonding of the ceramic coating to the substrate. The bonding can sometimes be further improved by pre-etching the surfaces to be coated.

The coating compositions may be applied to the substrates in any suitable manner, such as by spraying, swabbing, or brushing, to form coatings having the desired thickness, generally a thickness of up to about 1000 micrometers, frequently a thickness of about 10-250 micrometers. A coating of a desired thickness can be achieved by applying a single coating of that thickness or by applying the precursor polymer composition in multiple thinner layers, e.g., by applying the coating composition in layers of about 25-100 micrometers, each layer being dried by driving off the solvent before the next layer is applied.

When temperatures as high as about 200°-250° C. are used to drive off high boiling solvents, some pyrolysis of the preceramic polymer is initiated during the drying of the coating composition. However, higher temperatures, i.e., about 675°-900° C., preferably about 825°-875° C., are required to convert the preceramic coating to a ceramic coating. This pyrolysis may be delayed until the final desired thickness of preceramic coating has been deposited. However, it is generally preferred to pyrolyze each one or two layers of dried preceramic coating before applying the next layer of coating composition. The time required for the pyrolysis is generally about 1-60 minutes, depending on the particular pyrolysis temperature selected. In the preferred embodiment of the invention where the coating is applied in multiple layers, each one or two of which is pyrolyzed before the application of the next layer, and the pyrolysis temperature is about 825°-875° C., it is generally preferred to pyrolyze the first coat for only about five minutes and then to pyrolyze subsequent coats for longer times up to about 15 minutes.

When the coating is intended to protect a substrate from oxidative deterioration at very high temperatures, e.g., temperatures higher than 800° C., each pyrolysis is followed by thermal treatment of the coated substrate at about 1075°-1250° C., preferably about 1100°-1175° C., most preferably about 1125° C., in an atmosphere containing not more than a minor amount of oxygen, e.g., in a nitrogen, argon, or helium atmosphere, to convert the ceramic coating into a homogeneous film. This treatment may be accomplished by raising the temperature in the vessel used for the pyrolysis or by transferring the coated substrate to a vessel maintained at the higher temperature; and it is preferably continued for about five minutes for the first coat and longer periods, e.g., about 15-20 minutes, for subsequent coats.

After each pyrolysis or pyrolysis/heat treatment step employed in providing a ceramic coating, the coated substrate is cooled. Optimum results are attained when this cooling is accomplished at a rate not greater than about 50° C./minute, preferably about 20°-30° C./minute, until the substrate temperature is below 500° C., at which time further cooling may be accomplished at ambient air temperature.

Although not essential, it is preferred to keep the starting polysilazane and the organoborosilazane polymers and compositions formed from it in a dry atmosphere until a layer of ceramic has been formed because of the susceptibility of the preceramic materials to attack by water and other compounds having active hydrogens.

As already indicated, the organoborosilazane polymer dispersions of the invention are useful in preparing a variety of ceramic objects, but the major advantage of the invention is its provision of compositions capable of protecting normally oxidizable materials from oxidative deterioration at elevated temperatures. This advantage is of particular importance in the protection of carbon/carbon composites, graphite, and metals used in aerospace applications, such as engine components, advanced nozzle system components, and high-temperature vehicle structures.

The following examples are given to illustrate the invention and are not intended as a limitation thereof.

EXAMPLE I Synthesis of Polysilazane Part A

A suitable reaction vessel was charged with 14 L of anhydrous tetrahydrofuran and cooled to about 0° C., after which 1545 g (13.43 mols) of methyldichlorosilane was added to the vessel, and stirring at about 60 rpm was begun. A slow steady stream of 1058 g (62.12 mols) of anhydrous ammonia gas was introduced into the vessel at a flow rate such that the reaction pressure was maintained at or below 400 kPa, and the reaction temperature stayed in the range of 0°-10° C. Then the reaction mixture was stirred at 0° C. for about three hours, after which the coolant flow on the vessel was shut off, and the system was put under gentle nitrogen purge to allow the reaction mass to warm to room temperature and the majority of the excess ammonia to vent off. Then the reaction vessel was pressurized with sufficient nitrogen gas to pump the product mass through a bag filter assembly into a holding tank, where it was verified that the filtrate solution was free of particulates.

Part B

The clear filtrate from Part A was discharged into a polymerization vessel and chilled to about 0° C., and a suspension of 3.6 g (0.089 mol) of potassium hydride powder in about 100 mL of anhydrous tetrahydrofuran was added to begin the polymerization reaction. The reaction mixture was maintained at 0° C. for about 8 hours and then allowed to warm gradually to about 22° C. After a total of about 26 hours of polymerization at 0°-22° C., the reaction was quenched by adding about 12.6 g (0.13 mol) of dimethylchlorosilane to the polymerization solution.

The polymer product was isolated by (1) concentrating the product solution to about 4 L of volume by vacuum distillation, (2) centrifuging the concentrated solution to obtain a clear supernatant solution and a white precipitate, (3) decanting off the supernatant solution from the precipitate, and (4) flashing off the volatiles from the supernatant solution by vacuum distillation to provide a white solid. Proton NMR spectra of the polymer in deuterated chloroform solvent had resonances consistent with those reported in Seyferth et al.-II for polysilazane and with the presence of a small amount, i.e., 2.4% by weight, of residual tetrahydrofuran.

EXAMPLE II Synthesis of organoborosilazane polymer

A clear solution of four parts by weight of trimethoxyboroxine in a mixture of 0.2 part by weight of xylene and 0.6 part by weight of 1-methyl-2-pyrrolidone was slowly added to a clear solution of one part by weight of the polysilazane of Example I in a mixture of 0.6 part by weight of xylene and 1.8 parts by weight of 1-methyl-2-pyrrolidone. An exothermic reaction occurred to form a solution of an organoborosilazane polymer.

EXAMPLE III Preparation of Dispersion

A mixture of 0.3 part by weight of aluminum-silicon eutectic, 1.7 parts by weight of o-silicon carbide, two parts by weight of silicon hexaboride, and 1.8 parts by weight of amorphous silicon metal was homogenized and vacuum-dried for at least two hours, after which the organoborosilazane polymer solution of Example II was added and intimately mixed with it to form a dispersion.

EXAMPLE IV

Inhibited carbon/carbon composite coupons (i.e., carbon/carbon composite coupons containing an oxidation inhibitor) having nominal dimensions of about 2.5 cm×2.5 cm×0.34 cm were cleaned, abraded, cleaned again, etched with nitric and hydrofluoric acids, recleaned, dried, thoroughly swab-coated in an inert atmosphere with the dispersion of Example III, dried, heated at 825°-875° C. to pyrolyze the coating to form a ceramic and then at 1100°-1175° C. to convert the ceramic coating to a homogeneous film, cooled at a rate of about 20°-30° C./minute until the substrate temperature was below 500° C., and then allowed to cool to room temperature under ambient air conditions. After the coated coupons had been cooled, they were provided with three additional ceramic coats in essentially the same manner to provide coupons having a coating thickness of about 0.08-0.1 mm.

The effectiveness of the ceramic coats thus obtained in protecting the substrates from oxidation was determined by an oxidation test. The coated specimen was mounted horizontally in a half section of a silicon carbide tube which was used as a holder and which allowed over 99% of the coupon surface to be directly exposed to hot ambient convecting air. The holder and specimen were placed in a box furnace which had been preheated to about 1095° C. and held there for 24 hours, after which the holder and specimen were removed from the furnace and quenched in ambient air, and the cooled specimen was weighed. The weight loss on oxidation was determined to be only 18-20% after 24 hours. This compares with an oxidation weight loss of 100% after 24 hours when the uncoated inhibited carbon/carbon composite coupons were subjected to the same oxidation test.

EXAMPLE V

Example IV was essentially repeated except that inhibited carbon/carbon composite coupons containing a different oxidation inhibitor were used. The oxidation weight loss after 24 hours was only 28-38%, compared with an oxidation weight loss of 100% after 24 hours when the uncoated inhibited carbon/carbon composite coupons were subjected to the same oxidation test.

It is obvious that many variations can be made in the products and processes set forth above without departing from the spirit and scope of this invention. 

What is claimed is:
 1. A dispersion of about 0-3 parts by weight of aluminum-silicon eutectic, about 0-4 parts by weight of silicon carbide, about 1.5-5 parts by weight of silicon boride, and about 0.4-5 parts by weight of silicon metal in a solution of an organoborosilazane polymer in an organic solvent, the organoborosilazane polymer being the product obtained by reacting about 0.25-20 parts by weight of a trialkoxy-, triaryloxy-, or tri(arylalkoxy)boroxine with one part by weight of a polysilazane.
 2. The dispersion of claim 1 wherein the silicon carbide is α-silicon carbide.
 3. The dispersion of claim 1 wherein the silicon boride is silicon tetraboride.
 4. The dispersion of claim 1 wherein the silicon boride is silicon hexaboride.
 5. The dispersion of claim 1 wherein the silicon metal is amorphous.
 6. The dispersion of claim 1 wherein the boroxine is a trialkoxyboroxine in which the alkoxy groups contain 1-6 carbons.
 7. The dispersion of claim 6 wherein the boroxine is trimethoxyboroxine.
 8. The dispersion of claim 1 wherein the polysilazane is a polymer prepared by reacting an organodihalosilane with ammonia, treating the ammonolysis product with a basic catalyst which is capable of deprotonating an NH group that is adjacent to an SiH group, and quenching the resultant product with an electrophilic quenching reagent.
 9. The dispersion of claim 8 wherein the organodihalosilane is methyldichlorosilane and the basic catalyst is potassium hydride.
 10. A process for preparing the dispersion of claim 1 which comprises (A) adding about 0.25-20 parts by weight of a trialkoxy-, triaryloxy-, or tri(arylalkoxy)boroxine to a solution of one part by weight of a polysilazane in an organic solvent and agitating the resultant reaction mixture at a temperature below 50° C. for a period of time sufficient to form an organoborosilazane polymer solution, (B) homogenizing and drying a powdered mixture of about 0-3 parts by weight of aluminum-silicon eutectic, about 0-4 parts by weight of silicon carbide, about 1.5-5 parts by weight of silicon boride, and about 0.4-5 parts by weight of silicon metal, and (C) adding the organoborosilazane polymer solution to the homogenized mixture and agitating to form a uniform dispersion.
 11. The process of claim 10 wherein the homogenized mixture is uniformly dispersed in an organic medium prior to the addition of the organoborosilazane polymer solution.
 12. An article which comprises a substrate that is normally susceptible to oxidative deterioration and a coating derived from the dispersion of claim
 1. 13. An article which comprises a substrate that is normally susceptible to oxidative deterioration and a ceramic coating provided by (1) coating the substrate with the dispersion of claim 1, (2) drying the coating, (3) heating the coated substrate in an inert atmosphere at a pyrolysis temperature of about 675°-900° C. until the polymer is converted to a ceramic, (4) subjecting the ceramic-coated substrate to thermal treatment at about 1075°-1250° C. until the ceramic coating has been converted to a homogeneous film, (5) gradually cooling the thus-coated substrate, and (6) repeating the coating, drying, pyrolysis, heat treatment, and cooling steps one or more times when a thicker coating is desired.
 14. The article of claim 13 wherein the substrate is a porous substrate that is infiltrated during the coating operation.
 15. A ceramic derived from the dispersion of claim
 1. 